Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1999-11-23
2002-04-09
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S040000
Reexamination Certificate
active
06367909
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for reducing drop placement error of ink drops emitted by printheads, e.g. acoustic ink printing (AIP) printheads, in printers. Ink types include aqueous and phase change (hot melt), with finite electrical conductivity to allow inductive charging at drop emission. More particularly, two schemes are contemplated to facilitate reduction of drop placement error, preferably to zero, for printing on print medium. In both schemes, segmented counter electrodes are biased at iteratively predetermined voltages and located across a print gap from drop ejectors integrated in the printheads. For printing on stationary medium such as paper on a belt or platen, absolute drop placement error for each ejector row is maintained to be zero to obtain the required bias electrode voltages. For printing on moving medium such as paper on a drum, the same time of flight for all ejector rows referenced to the 1
st
row is maintained, thus resulting in the relative drop placement error being zero and the absolute error being negligibly small.
While the invention is particularly directed to the art of drop placement in the context of acoustic ink printing where the print medium is or is disposed on a curved surface, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, the invention may be used to print on planar surfaces as well as in a variety of ink jet printing applications.
By way of background and generally, in many printing applications, drops of ink are ejected or emitted on demand and deposited onto a medium to form the printed image. For high resolution print images, a combination of small drop size and precise drop placement are necessary to ensure good image quality. The requirement for accurate drop placement is especially critical for color printing on moving non-planar print media such as drums or belts.
More specifically, acoustic ink printing involves the emission of a droplet of ink from a pool of ink toward a print medium. Acoustic waves are generated and focussed toward the surface of the ink pool to emit the droplet therefrom. While acoustic ink printing elements may take various forms, such elements typically include a piezoelectric transducer to generate the acoustic waves, a lens to focus the waves at the surface of the ink pool, a cover plate with apertures formed therein to allow emission of the ink, and corresponding wiring. It is to be appreciated that approximately one thousand (1,000) or more of these elements may be disposed on a single printhead in a variety of configurations. Typically, however, the printing elements are formed in eight rows along the length of the printhead. Acoustic ink printing systems are disclosed, for example, in U.S. Pat. Nos. 4,308,547; 4,697,195; 5,028,937; and 5,087,931, all of which are incorporated herein by reference.
The advantages of electrostatic field acceleration in reducing drop placement errors for both aqueous and phase change AIP efforts are known. Errors in drop placement due to transverse disturbances, such as airflow and skewed drop ejection, are reduced by providing a Coulomb force component normal to the printing surface to attract the drops. This force also acts to overcome drag, thus providing an impetus for the drop to move across the print gap. Otherwise, the drop may decelerate and fall back onto the print head leading to contamination problems that adversely affect print head reliability and lifetime. Another important advantage is the reduction in mechanical energy for drop ejection by supplying just sufficient energy for drop formation; and then using the electrostatic field to accelerate the drop. This measure results in significant reduction of power.
U.S. Pat. Nos. 4,386,358 and 4,379,301 to Fischbeck, which are commonly assigned and incorporated herein by reference, disclose a method for electrostatically deflecting electrically charged ink drops emitted from an ink jet printhead. Charges placed on electrodes on the printhead disclosed by Fischbeck are controlled to steer the charged ink drops in desired directions to compensate for known printhead movement. By electrostatically steering the charged ink drops, the method disclosed in Fischbeck compensates for ink drop misdirection caused by the known printhead movement when the ink drop is emitted.
However, the electrostatic deflection method disclosed by Fischbeck does not compensate for unpredictable environmental factors that can affect ink drop trajectories. Such environmental factors include air currents and temperature gradients between the printhead and the print substrate. In acoustic ink jet printheads, unpredictable variations in the dynamics of ink drop creation also detrimentally affect ink drop trajectories. Some of the variations in ink drop creation are caused by aberrations in the lithography of Fresnel lens which are in some embodiments used to focus the acoustic wave used to create the ink drops.
U.S. Pat. No. 5,975,683 entitled Electric-Field Manipulation of Emitted Ink Drops in Printing, which is commonly assigned, and is hereby incorporated by reference, discloses the use of an electric field to reduce droplet misdirectionality, by inducing a charge on a drop as it breaks off from the bulk of the fluid. The charged drop is then accelerated into the paper, by holding the paper at a relatively large potential (this same potential may be used to induce the charge on the drop). The application teaches selectively deflecting the ink drops slightly to enhance the resolution of the image produced by a given printhead configuration. The ink jet actuators form and impart an initial velocity on the ink drops. The charged ink drops are then steered by electrodes such that the drops alternately impact upon the print medium at positions slightly offset from positions directly opposite the apertures of the printhead.
This approach, though useful, has drawbacks. It requires large voltages, of the order of 1 to 2 kV. Also, it will suffer from many of the same imaging artifacts as occur in ionographic printing, where because charge is deposited onto the printing substrate, there is print-dependent interaction of the accelerating field with the charged drop. That is, as drops are accumulated on the paper, so is their charge. If this charge is not removed quickly enough, it will produce a print-dependent potential at the paper surface, which will interfere with the acceleration of subsequent drops. Finally, the acceleration expected for drops under typical print conditions is only large enough to reduce the misplacement of drops by some 50% at the paper surface, so that the correction of the misdirection, while significant, is not complete.
U.S. patent application Ser. No. 08/721,290 (filed Sep. 26, 1996) entitled Method and Apparatus for Moving Ink Drops Using an Electric Field, which is commonly assigned, and is hereby incorporated by reference, discloses using an electric field to charge and impart a force onto ink drops to control for motion of the ink drops, including biasing the print support medium with a voltage source.
U.S. patent application Ser. No. 09/098,763 (filed Jun. 17, 1998) entitled “Reduction of Spot Misplacement Through Electrostatic Focusing of Uncharged Drops”, which is commonly assigned and hereby incorporated herein by reference, is directed to lateral focus of aqueous ink drops onto a substrate through implementation of electric fields for use in acoustic ink printing.
Known techniques do not take into account, however, that the print medium may be non-planar, e.g. comprised of a curved surface. These techniques only effectively contemplate the placement of drops on a planar medium. This is significant because the geometry of the print medium presents an additional complicating source for drop placement error. Addressing the problems associated with printing on a curved surface is particularly important in high volume printing systems where drums ar
Fay Sharpe Fagan Minnich & McKee LLP
Pham Hai
Xerox Corporation
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